LA - drafted Kobe (franchise player) and built around him.
MIA - drafted Wade (franchise player) and built around him.
Mavs - drafted Dirk (franchise player) and built around him.
Bulls - drafted Rose (franchise player) and built around him.
OKC - drafted Durant and Westbrook (franchise players).
Spurs - drafted 3 superstars (Duncan, Ginobili, Parker).
The thing about this is the Mavs and the Bulls were bad for a number of years before they drafted Dirk and Rose and both teams had multiple top 5 picks in those years. Yet neither team was on the path to winning before drafting these players. They could have been winning 55 or 25 games in the years leading up to those drafts without affecting their futures. Getting bad and being bad in and of itself doesn't get you close to a title. It can be a long, drawn out journey back to where you think we are now, a team that can be good but won't contend. It's not a sure path to the top and it's not the only path to the top. It's really a last resort.
All of those teams had to get bad and draft those franchise players in order to become relevant. Only the Lakers, who lured Shaq in free agency because they are in L.A., were able to become a dominant team without doing a lot of good drafting.
Getting bad and drafting high doesn't guarantee that you will become a contender, but it's the best available option.
Otherwise, you have to take a middling path and hope that one or more of your 10-20 picks will turn into an All-Star and you will be able to lure more All-Stars in free agency, or trade second-tier prospects for them.
The worst option, I think, is to clear cap space and think that you can convince a superstar free agent or two to come play for your team. If you are in NY or LA, perhaps, you can hope for that strategy to work. Not so much in Boston.
None of these methods of rebuilding guarantees that your team will become a contender in any short amount of time. Choosing to stay competitive as possible and just hope for the best does guarantee that your team will at least be interesting to watch most of the time and that you'll have a fighting chance of making the playoffs and winning a series or two in the immediate future. There's a much greater chance of simply wallowing in the middle of the pack for years and years that way, though, even if it keeps fans interested and makes money.
The franchises that are well managed and have money tend to make good moves and take advantage of teams that don't have money. For those franchises, the rebuilding period is often not as long. The Celtics are generally well managed and have lots of money, with a big market, so I think there's a good chance that rebuilding with high draft picks is a viable option.
This isn't the NFL where anybody has the chance to win it all if they get hot at the right time. Teams that win it all are almost exclusively ones that draft superstars and make the right moves to build around them.
Exceptions do occur now and then, like the '04 Pistons, but those are the exceptions that make the rule. Aiming to win by being like the '04 Pistons is not a strategy that's likely to result in championships.